Re: [PATCH v3 13/25] KVM: TDX: Fold tdx_mem_page_record_premap_cnt() into its sole caller

["Huang, Kai" <kai.huang@intel.com>]

On Thu, 2025-10-16 at 17:32 -0700, Sean Christopherson wrote:
> Fold tdx_mem_page_record_premap_cnt() into tdx_sept_set_private_spte() as
> providing a one-off helper for effectively three lines of code is at best a
> wash, and splitting the code makes the comment for smp_rmb()  _extremely_
> confusing as the comment talks about reading kvm->arch.pre_fault_allowed
> before kvm_tdx->state, but the immediately visible code does the exact
> opposite.
> 
> Opportunistically rewrite the comments to more explicitly explain who is
> checking what, as well as _why_ the ordering matters.
> 
> No functional change intended.
> 
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Kai Huang <kai.huang@intel.com>

> ---
>  arch/x86/kvm/vmx/tdx.c | 49 ++++++++++++++++++------------------------
>  1 file changed, 21 insertions(+), 28 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 6c0adc1b3bd5..c37591730cc5 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1605,29 +1605,6 @@ static int tdx_mem_page_aug(struct kvm *kvm, gfn_t gfn,
>  	return 0;
>  }
>  
> -/*
> - * KVM_TDX_INIT_MEM_REGION calls kvm_gmem_populate() to map guest pages; the
> - * callback tdx_gmem_post_populate() then maps pages into private memory.
> - * through the a seamcall TDH.MEM.PAGE.ADD().  The SEAMCALL also requires the
> - * private EPT structures for the page to have been built before, which is
> - * done via kvm_tdp_map_page(). nr_premapped counts the number of pages that
> - * were added to the EPT structures but not added with TDH.MEM.PAGE.ADD().
> - * The counter has to be zero on KVM_TDX_FINALIZE_VM, to ensure that there
> - * are no half-initialized shared EPT pages.
> - */
> -static int tdx_mem_page_record_premap_cnt(struct kvm *kvm, gfn_t gfn,
> -					  enum pg_level level, kvm_pfn_t pfn)
> -{
> -	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> -
> -	if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
> -		return -EIO;
> -
> -	/* nr_premapped will be decreased when tdh_mem_page_add() is called. */
> -	atomic64_inc(&kvm_tdx->nr_premapped);
> -	return 0;
> -}
> -
>  static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>  				     enum pg_level level, kvm_pfn_t pfn)
>  {
> @@ -1638,14 +1615,30 @@ static int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
>  		return -EIO;
>  
>  	/*
> -	 * Read 'pre_fault_allowed' before 'kvm_tdx->state'; see matching
> -	 * barrier in tdx_td_finalize().
> +	 * Ensure pre_fault_allowed is read by kvm_arch_vcpu_pre_fault_memory()
> +	 * before kvm_tdx->state.  Userspace must not be allowed to pre-fault
> +	 * arbitrary memory until the initial memory image is finalized.  Pairs
> +	 * with the smp_wmb() in tdx_td_finalize().
>  	 */
>  	smp_rmb();
> -	if (likely(kvm_tdx->state == TD_STATE_RUNNABLE))
> -		return tdx_mem_page_aug(kvm, gfn, level, pfn);
>  
> -	return tdx_mem_page_record_premap_cnt(kvm, gfn, level, pfn);
> +	/*
> +	 * If the TD isn't finalized/runnable, then userspace is initializing
> +	 * the VM image via KVM_TDX_INIT_MEM_REGION.  Increment the number of
> +	 * pages that need to be mapped and initialized via TDH.MEM.PAGE.ADD.
> +	 * KVM_TDX_FINALIZE_VM checks the counter to ensure all mapped pages
> +	 * have been added to the image, to prevent running the TD with a
> +	 * valid mapping in the mirror EPT, but not in the S-EPT.
> +	 */
> +	if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE)) {
> +		if (KVM_BUG_ON(kvm->arch.pre_fault_allowed, kvm))
> +			return -EIO;
> +
> +		atomic64_inc(&kvm_tdx->nr_premapped);

Nit: the comment

  /* nr_premapped will be decreased when tdh_mem_page_add() is called. */

is lost.  I think we can somehow embed it to the big comment above?